Last comment not entirely accurate. Delete slice is sort of working now. IJ2 takes a 4x15 CZ stack and when deleting a slice turns it into a 4x14 stack (thus deleting 4 planes). This is likely just by accident.

Add slice is not working right now.

Probably best if we define our own plugins to replace a number of them in Image::Stacks menu (such as add slice, delete slice, next slice, prev slice, set slice).